The present invention relates to an ON-OFF timer circuit for use in a dc-dc converter. More specifically, the present invention relates to a boost dc-dc converter type for use in driving LEDs.
A conventional boost dc-dc converter is shown in FIG. 1A which is used for driving LEDs, and the signals observed in the boost dc-dc converter are shown in FIGS. 1B and 1C. As shown in FIG. 1B, dc-dc control 2 outputs the switching control signal to a driver circuit 1 before driving the switch 3 for the dc-dc operation. In this mode of operation, there is a possibility of the occurrence of sub-harmonic oscillations, which in turn may cause the dc-dc converter system to become unstable. These sub-harmonic oscillations occur if the following conditions are met:                Operating in continuous conduction mode;        Duty cycle is 50% or higher.According to a technical report on “Ramp Compensation for Current-Mode Converters” by Christophe Basso of ON Semiconductor, as published in the Power Electronics Technology magazine in July 2004, a resultant perturbation due to the sub-harmonic oscillations may be represented by the formula:ΔIL(n)=ΔIL(0)×(D/1−D)n where D=duty cycle;n=number of switching cycles;IL=inductor current;ΔIL=inductor current step as a result of the said perturbation.        
From the formula, we can see that if the duty cycle D is less than 50%, the perturbation will die out after several cycles. However, for duty cycles more than 50%, the perturbation continues to grow with every cycle. An exemplary waveform showing this phenomenon is as shown in FIG. 1C. In FIG. 1C, a typical inductor current under normal operating condition is given as IL0. Upon introduction of the said perturbation (ΔIL0), the resultant oscillating waveform is given by ILsh.
Conventional means to prevent the sub-harmonic oscillations would be to use the Slope Compensation Technique, as described in U.S. Pat. No. 4,837,495 (Current Mode Converter With Controlled Slope Compensation) by Zansky; as well as by using the Hysteretic Control, as described in U.S. Pat. No. 6,628,106 (Control Method and Circuit to Provide Voltage and Current Regulation for Multiphase DC/DC Converters) by Batarseh et al.
These two techniques, however, involve complex systems. The present invention aims to provide a solution to resolve the instability problems described above. Based on the present invention, a technique is introduced for controlling the ON and OFF widths of the output pulse-width modulated waves.